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Friday, 22 July 2011

Symbion

Image: P. Funch and R.M. Kristensen

This is a weird one. A very weird one.

In 1995 Danish scientists discovered tiny, worm-like creatures living on the mouthparts of the Norway lobster. This little worm proved to be extremely peculiar. It was placed in a brand new genus: Symbion, referring to its symbiotic relationship with the lobster.

But it went further. It was so unique, it was given an entirely new phylum all its own: Cycliophora.
There are just 36 phyla in the animal kingdom, and this particular one now contains three known species. One on the Norway lobster, one on the American lobster and one on the European lobster. They REALLY like lobsters. "Lobster and Symbion, sitting in a tree..."

Image: Peter Funch

Cycliophora comes from the Greek for "carrying a small wheel". Small? I guess the Greeks didn't have a word for microscopic? The wheel isn't an itsy-bitsy car or unicycle, it's actually the worm's mouth. They have a circular mouth lined with cilia. These hairs wave in the water to draw tiny crumbs toward them. Symbions don't seem to harm their hosts at all, they just eat up microscopic specks of food that the lobster produces as it chews on worms and fish. They have a U shaped gut so waste is blasted out, hopefully too far for it to go straight back into their mouth again. Hmmm... we really need to move on.

Beneath the mouth is a chunky body which ends in an adhesive disc that sticks onto tiny hairs and bristles of the lobster. The whole animal is only about 0.35 millimetres long, small enough that there can be hundreds on each lobster!

It gets even weirder when you take a look at reproduction. So let's do that!

Everything I described so far is a feeding adult. They eat, but they don't seem to be male or female. They mainly just eat, but they can also reproduce. They can create a Pandora larva that will grow into another feeding adult. This is good old fashioned asexual reproduction that will produce a clone of itself. But there are two other kinds of larvae they can produce.

When a feeding adult produces a female, it stays inside the adult and waits. Adults can also produce a Prometheus larva, who will swim along and attach itself to another feeding adult. This will later produce not one but 2 or 3 males, each with a nervous system, brain, muscles and cilia to help him swim to yet another feeding adult. He's only about 0.08 mm long and has no gut, but he seeks out a female to fertilise.

So, a female is released from her mother and is fertilised by a male. She now seeks out a safe place on the lobster she lives on and encysts. She never feeds, she just has a precious egg that must be allowed to grow.

In time, yet another larva hatches from the egg and emerges from the cyst. This is the chordoid larva, which will swim off to discover a whole new lobster, a whole new world to conquer. It is this that will finally develop into yet another feeding adult to complete the cycle and commence colonisation by creating clones of itself. They must be really happy to finally get a chance to eat again!

So there you have it. The Symbion may appear to be an unremarkable, microscopic sack stuck to a hair, but they hide an utterly extraordinary journey. It's like those little islands that require a plane, train, helicopter and rowing boat to get to. All to go from one lobster to another lobster.